The Red and Rec recombination pathways of phage lambda will be studied by isolating dimeric intermediates in the absence of DNA replication and encapsidation. Intermediates will be followed by hybrid density and ratio isotope labels when isolated in Cs-gradient from the lysate of an infection of 3H-Bu and 14C-Thy labelled parental phage. Finally, biparental origin will be confirmed by denaturation mapping. The frequency and topology of the dimers will be analyzed using mutations in different genes so that their roles can be understood (e.g., red, gam, RecA, RecBD, pol and lig). We shall also attempt to understand the mechanism of Chi stimulation of Rec recombination and whether the stimulation has polarity. Alternative approaches to isolate intermediates will be attempted in order to improve the yield and to preseve the proteins bound to DNA. These will involve the use of increased substrate and enzyme concentrations, psoralen-cross-linking of duplex DNA and HCHO cross-linking of DNA-protein complexes.